The instant invention relates to test fixtures for use in testing printed circuit boards on test computers and more particularly to a test fixture which is operative for contacting predetermined areas of a printed circuit board with increased accuracy and reliability in order to more effectively test the printed circuit board on a test computer.
A variety of different types of test fixtures have been heretofore available for testing printed circuit boards on test computers. In general, however, most of the previously available test fixtures have been of a type including a base, a probe plate mounted on the base, a plurality of spring loaded electrical contact probes mounted on the probe plate in a predetermined array, and a top plate mounted on the base in upwardly spaced relation to the probe plate. The top plate of a fixture of this type is normally adapted for receiving and positioning a predetermined printed circuit board thereon so that predetermined portions of the printed circuit board are aligned with the contact probes on the probe plate. Further, the top plate of a fixture of this type generally has a plurality of apertures therein which are aligned with the contact probes, and the top plate is normally adapted to be moved between a first position of predetermined spaced relation to the probe plate and a second position of more closely spaced relation thereto. More specifically, a test fixture of this type is generally adapted so that the top plate thereof is movable downwardly to a second position wherein the contact probes on the probe plate pass through the apertures in the top plate and electrically contact the predetermined areas on the printed circuit board in order to electrically connect the printed circuit board to a test computer electrically connected to the contact probes.
While it has been found that test fixtures of the above type are generally effective for testing printed circuit boards, it has been found that in some instances the contact probes of a fixture of the above type can be less than entirely effective for reliably and accurately contacting the appropriate areas of a printed circuit board received on the top plate thereof. This is because it is generally necessary for the spring loaded contact probes of a test fixture of the above type to extend a substantial distance, such as three quarter of an inch or more, from the probe plate thereof in order to pass through apertures in the top plate thereof and contact a printed circuit board received on the top plate. In this regard, it has generally been found that relatively minor manufacturing irregularities can cause the contact probes of a test fixture to improperly contact, or even miss, the appropriate areas of a printed circuit board during a test operation. These irregularities can be caused by radial play in the tip portions of the probes on a probe plate, misregistration errors between the top plate and the probe plate of a fixture or minor drilling or mounting errors which cause the test probes on a probe plate to be mounted at incorrect angles thereon. In any event, it has been found that these minor imperfections can frequently present problems in the accuracy with which the test probes of a test fixture are capable of reliably contacting a printed circuit board during a testing operation.
The instant invention provides a significant improvement in printed circuit board testing devices which virtually eliminates the problems which have been heretofore experienced with respect to accurately and reliably testing printed circuit boards with test fixtures. More specifically, the instant invention provides an improved printed circuit board test fixture which is not effected by minor irregularities in probe mounting, probe tolerances or probe plate registration. Still more specifically, the test fixture of the instant invention comprises a base, a probe plate mounted on the base, and a plurality of primary electrical contact probes on the probe plate which are disposed in a predetermined array and adapted to be electrically connected to a test computer. The test fixture further comprises a top plate mounted on the base in substantially parallel relation to the probe plate and a plurality of secondary electrical contact probes mounted on the top plate. The top plate is adapted so that it is operative for receiving and positioning a printed circuit board thereon, whereby predetermined portions of the printed circuit board are substantially aligned with predetermined primary contact probes on the probe plate. The top plate is further adapted so that it is movable between a first position of a predetermined spaced relation to the probe plate and a second position of more closely spaced relation thereto. The secondary electrical contact probes are mounted on the top plate so that they are disposed in the same predetermined array as the primary electrical contact probes, and the secondary probes are aligned with the primary contact probes. Further, the secondary contact probes include contact portions which are movable between first positions of spaced disengagement from the predetermined portions of the printed circuit board and second positions of engagement therewith. The primary contact probes are disposed in spaced relation to the secondary contact probes when the top plate is in the first position thereof, and the primary probes engage the secondary probes to move the contact portions thereof from the first positions thereof to the second positions thereof when the top plate is moved from the first position thereof to the second position thereof. Accordingly, when the top plate is moved downwardly from the first position thereof to the second position thereof, the secondary contact probes are moved into engagement with the primary contact probes to move the contact portions of the secondary probes into engagement with the predetermined portions of a printed circuit board received on the top plate.
The secondary probes preferably comprise sleeve portions which are mounted in the top plate, and the contact portions of the contact probes preferably travel in the sleeve portions between the first and second positions thereof. The contact portions are preferably freely slidable in the sleeve portions thereof, but they are preferably gravitationally biased toward the first positions thereof. Further, the secondary contact probes are preferably adapted so that the contact portions thereof travel within predetermined limits of travel in the sleeve portions thereof. Each of the contact portions preferably includes a pointed contact head which is engagable with a printed circuit board received on the top plate, and each of the contact portions preferably further includes an enlarged rear end portion which is adapted to be engaged by the primary contact aligned therewith for moving the contact portions of the secondary contacts between the first and second positions thereof.
It has been found that the test fixture of the instant invention represents a significant improvement over the heretofore available test fixtures as far as eliminating the problems of accurately electrically connecting predetermined portions of a printed circuit board to a test computer. Specifically, it has been found that because the primary contact probes are merely required to contact the enlarged rear end portions of the secondary contact probes rather than being required to pass through apertures in the top plate and then directly contact a printed circuit board, the relative distances traveled by the top plate, as it is moved to the second position thereof, is greatly reduced so that the effects of minor imperfections in probe angularity, probe construction or top plate registration are minimized. Further, because the contact portions of the secondary contact probes are merely required to travel relatively short distances to contact a printed circuit board, minor imperfections in probe angularity or construction with respect to the secondary contact probes are of relatively minor significance.
Accordingly, it is primary object of the instant invention to provide an improved test fixture for reliably and accurately electrically connecting a printed circuit board to a test computer.
Another object of the instant invention is to provide an improved test fixture which is not significantly effected by minor irregularities in contact probe construction, contact probe mounting or top plate alignment.
Another object of the instant invention is to provide an improved test fixture for testing a printed circuit board which comprises a top plate having a plurality of secondary contact probes mounted therein for reliably contacting predetermined areas of a printed circuit board.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.